Controlling the attitude of a vehicle is an important component of any autopilot or similar control system that acts both to sense and maintain the orientation of a vehicle according to user-defined conditions. Typically an Inertial Measurement Unit (IMU) is employed as a key component of any such control system. These devices rely on measurements of changes in the inertial characteristics of a vehicle to determine the vehicle's orientation. However, IMU's suffer from a number of disadvantages. An IMU typically requires a number of individual sensors to be operating, some of which can include rapidly moving parts, in order to provide sensible information on vehicle attitude. Each of these sensors requires its own detailed calibration and tuning and as a consequence IMU's are relatively expensive devices to purchase and maintain.
Another inherent disadvantage of IMU type systems occurs when a vehicle is subjected to violent turbulence and therefore is rapidly maneuvering. In these conditions it becomes increasingly difficult to distinguish the direction of the gravity vector and hence direction from externally imposed accelerations including centripetal accelerations. These types of conditions become more pronounced as the size of the vehicle reduces and hence aircraft such as Unmanned Aerial Vehicles (UAV) require more sophisticated and ruggedised IMU's to compensate for these effects. As would be expected these higher performance IMU's add a significant cost penalty.
One attempt to provide a system that corrects attitude for an UAV which does not require an IMU is described in U.S. Pat. No. 5,168,152 entitled “Attitude Control with Sensors of a Mining Vehicle”. This document describes a control system for a vehicle which includes Ultra-Violet (UV) light sensors positioned on the vehicle. These sensors are connected together to produce a signal which is a function of the attitude of the vehicle. This signal is then compared with a command attitude signal to produce a difference signal which is a measure of the difference between the attitude of the vehicle and the attitude determined by the command attitude signal.
The principle behind this and similar attitude control systems can be readily explained by reference to FIGS. 1(a) and (b) which depict top and front views of an aircraft 10 including a left looking radiation sensor 1 and its associated field of view and a forward looking radiation sensor 2 and its respective field of view. Similarly, there is shown a right looking sensor 3 and an aft looking sensor 4. In the vertical plane the radiation sensors are arranged to look outwards with the centre of the field of view aligned with the horizon as shown in FIG. 1(a). FIG. 1(b) shows the alignment and representative size of the lateral fields of view 6, 7 corresponding to the right looking sensor 3 and left looking sensor 1 respectively.
Referring now to FIGS. 2(a), (b) and (c), aircraft 10 including radiation sensors as shown in FIGS. 1(a) and (b) is depicted flying towards the observer out of the page. Lateral fields of view 6 and 7 are also shown. Also to the left and right of aircraft 10 are representations of the expected positions of the horizon which directly relate to measured radiation sensor intensity. FIG. 2(a) illustrates left 30 and right 20 lateral views as would be expected as aircraft 10 rolls left. Similarly FIG. 2(b) indicates the left 31 and right 21 lateral views when aircraft 10 rolls right. Clearly the lateral sensor in the direction of roll will sense more “dark” ground and hence have a lower output. When aircraft 10 is not experiencing any roll as shown in FIG. 2(c) the left 32 and right 22 views are substantially equivalent resulting in equivalent intensities being measured by each sensor.
Thus output from the lateral radiation sensors may be used to give an indication of roll and furthermore appropriate control signals can be applied to controlling elements of the aircraft to cause it to orient itself so as to equalise the light intensities on either side of the aircraft 10. The same principle may be applied to the pitch axis, by incorporating measurements from longitudinal fore 2 and 4 aft radiation sensors. By balancing the measured output from these sensors the pitch of aircraft 10 can be reduced to level. Clearly the combination of lateral and longitudinal looking radiation sensors would allow full stabilization of attitude in pitch and roll.
As described in U.S. Pat. No. 5,168,152 typically UV sensors are used. This is due to the increased contrast in intensity between the sky and the ground at wavelengths ranging from blue (450 nm) to UV. This can be compared to measurements in the green to red part of the electromagnetic spectrum, where radiation emanated from these regions is of approximate equal intensity. Interestingly, for wavelengths in the near infrared (800 nm) and extending into the millimetre wave band, where the ground appears warm and intense and the sky appears cold and dark, similar contrasts in measured intensity may be measured.
The approach of using UV or single band radiation sensors suffers from a serious disadvantage which can be readily appreciated by reference to FIG. 3 which depicts aircraft 10 and representations of the lateral views of radiation sensors similar to that depicted in FIG. 2. In this example left lateral view 30 includes the sun, which is a significant source of UV radiation. This increases the overall intensity measured by the left viewing sensor of this region of sky with the net effect that in order to balance this, the aircraft will adjust roll so that right radiation sensor will measure an equal intensity thus causing the aircraft to roll left. Effectively, the sun is applying a bias to any roll calculation. This is shown schematically in FIG. 3 where the roll command function 50 is directly related to the difference 40 between left and right sensors having left 7 and right 6 lateral fields of view. Given the approximately 2° angular sub tense of the sun, roll errors of up to 60° are expected for systems employing measurements in the UV wavelength range. Obviously, the exact same problem will occur in the pitch axis for fore and aft radiation sensors.
One attempt to address this significant problem of attitude bias relies on the fact that the difference in brightness between the sun and the surrounding sky is dependent on wavelength and ranges from a factor of 10 in millimetre wave bands, 100 in thermal bands, to 1000 in visible bands. Thus the effect of attitude bias introduced by the sun can be mitigated somewhat to approximately 20° by employing thermal band sensors or down to 6.5° for millimetre wave measurements. However, at different wavelengths cloud cover will have a more pronounced effect also appearing as an electromagnetic source and resulting in another potential source of error causing an attitude biasing effect similar to that of the sun.
Another means to attempt to address attitude bias is to narrow the field of view of the radiation sensor to reduce the effect. However, this leads to an attitude control system having less capability to recover from unusual attitudes, and also more prone to biases due to local horizon deformations (such as trees), and obviously catastrophic failure if the sun should fall within the field of view of any of the radiation sensors.
It is therefore an object of the invention to provide a method and apparatus which improves the performance of attitude stabilisation systems which employ radiation sensors.